The present invention relates to methods in the field of recombinant DNA technology, and products related thereto. More particularly, the invention relates to methods and products for modulating the expression of exogenous genes in mammalian systems.
The steroid/thyroid hormone receptors comprise a superfamily of ligand-dependent transcription factors that play a crucial role in mediating changes in cell fate and function (Evans, R. M., Science 240:889-895 (1988)). The receptors transduce extracellular hormonal signals to target genes that contain specific enhancer sequences referred to as hormone response elements (HREs) Evans, (1988); Green and Chambon, Trends Genet. 4:309-314 (1988); Yamamoto, K. R., Annu. Rev. Genet. 19:209-252 (1985)). Each receptor recognizes its own HRE, assuring that a distinct response is triggered by each hormonal signal. Together the collection of related transcription factors and their cognate response elements provides a unique opportunity to control gene expression.
The DNA binding domain of each member of the steroid/thyroid hormone superfamily of receptors has 66-68 amino acids. Twenty of these, including nine cysteines, are conserved throughout the family. The modular structure of members of this receptor superfamily allows the exchange of homologous domains between receptors to create functional chimeras. This strategy was used to demonstrate that the DNA binding domain is solely responsible for the specific recognition of the HRE in vivo (Green and Chambon, Nature 325:75-78 (1987); Giguxc3xa8re et al., Nature 330:624-629 (1987); Petkovich et al., Nature 330:444-450 (1987); Kumar et al., Cell 51:941-951 (1987); Umesono et al., Nature 336:262-265 (1988); Thompson and Evans, Proc. Natl. Acad. Sci. U.S.A. 86:3494-3498 (1989) and in vitro (Kumar and Chambon, Cell 55:145-156 (1988)). By analogy with the proposed structure for Xenopus transcription factor IIIA (Miller et al., EMBO J. 4:1609-1614 (1985)), the invariant cysteines are thought to form two xe2x80x9czinc fingersxe2x80x9d that mediate the DNA binding function (Hollenberg and Evans, Cell 55:899-906 (1988)). Involvement of these cysteines in Zn(II) coordination is supported by extended X-ray absorption fine structure (Freedman et al., Nature 334:543-546 (1988)), and DNA binding by point mutagenesis experiments (Hollenberg and Evans, (1988)); Severne et al., EMBO J. 7:2503-2508 (1988)).
The HREs are in fact structurally related but functionally distinct. The glucocorticoid receptor response element (GRE), estrogen receptor response element (ERE), and thyroid hormone receptor response element (TRE) have been characterized in detail. These particular response elements have been found to have a palindromic pair of hexameric xe2x80x9chalf-sitesxe2x80x9d (Evans, (1988); Green and Chambon, (1988)). With optimized pseudo- or consensus response elements, only two nucleotides per half-site differ between GRE and ERE (Klock et al., Nature 329:734-736 (1987)). On the other hand, EREs and TREs have identical half-sites but the number of nucleotide spacers between the two half sites is different (Glass et al., Cell 54:313-323 (1988)).
In contrast to response elements having the palindromic sequence motif, the following hormone receptors typically recognize response elements having two half-sites in a direct-repeat (DR) sequence motif: RXR, RAR, COUP-TF, PPAR, and the like (see, e.g., Mangelsdorf et al., The Retinoids: Biology, Chemistry, and Medicine, 2nd Edition, Raven Press, Ltd., New York, 1994, Chapter 8). Thus at least three distinct means are used to achieve HRE diversity: 1) binding site specificity for a particular half-site; 2) nucleotide spacing between the two half-sites; and 3) the orientation of the half-sites to one another.
In insect systems, a pulse of the steroid hormone ecdysone triggers metamorphosis in Drosophila melanogaster showing genomic effects, such as chromosomal puffing, within minutes of hormone addition. Mediating this response in insects is the functional ecdysone receptor, a heterodimer of the ecdysone receptor (EcR) and the product of the ultraspiracle gene (USP) (Yao et al. (1993) Nature 366, 476-479; and Yao et al. (1992) Cell 71, 63-72). Responsiveness to an insect ecdysteroid can be recreated in cultured mammalian cells by co-transfection of EcR, USP, an ecdysone responsive reporter, and treatment with ecdysone or the synthetic analog muristerone A.
In the field of genetic engineering, precise control of gene expression is an invaluable tool in studying, manipulating and controlling development and other physiological processes. For example applications for regulated gene expression in mammalian systems include inducible gene targeting, overexpression of toxic and teratogenic genes, anti-sense RNA expression, and gene therapy (Jaenisch, R. (1988) Science 240, 1468-1474). For cultured cells, glucocorticoids and other steroids have been used to induce the expression of a desired gene.
As another means for controlling gene expression in a mammalian system, an inducible tetracycline regulated system has been devised and utilized in transgenic mice, whereby gene activity is induced in the absence of the antibiotic and repressed in its presence (see, e.g, Gossen et al. (1992) Proc. Natl. Acad. Sci. 89, 5547-5551; Gossen et al. (1993) TIBS 18, 471-475; Furth et al. (1994) Proc. Natl. Acad. Sci. 91, 9302-9306; and Shockett et al. (1995) Proc. Natl. Acad. Sci. 92, 6522-6526). However, disadvantages of this system include the continuous treatment of tetracycline to repress expression and the slow clearance of antibiotic from bone which interferes with quick and precise induction. While this system has been improved by the recent identification of a mutant tetracycline repressor which acts conversely as an inducible activator, the pharmacokinetics of tetracycline may hinder its use during development when a precise and efficient xe2x80x9con-offxe2x80x9d switch is essential (Gossen et al. (1995) Science 268, 1766-1769).
Accordingly, there is a need in the art for improved methods to precisely modulate the expression of exogenous genes in mammalian subjects.
In accordance with the present invention, there are provided various methods for modulating the expression of an exogenous gene in a mammalian subject. The invention method is useful in a wide variety of applications where inducible in vivo expression of an exogenous gene is desired, such as in vivo therapeutic methods for delivering recombinant proteins into a variety of cells within a patient.
Unlike prior art tetracycline based strategies, transferring ecdysone responsiveness to mammalian cells takes advantage of a naturally evolved steroid inducible system. Advantages of ecdysteroid use include the lipophilic nature of the compounds (which provides efficient penetrance thereof into all tissues, including the brain), short half-lives (which allow for precise and potent inductions), and favorable pharmacokinetics that prevent storage and expedite clearance.
In accordance with another embodiment of the present invention, there are provided modified ecdysone receptors, which can be in the form of homodimeric species or heterodimeric species comprising at least one silent partner of the steroid/thyroid hormone superfamily of receptors, along with an invention modified ecdysone receptor. Invention modified ecdysone receptors are useful, for example, in methods for modulating expression of an exogenous gene in a mammalian subject.
In accordance with additional embodiments of the present invention, there are provided nucleic acids encoding invention modified ecdysone receptors, modified ecdysone receptor response elements, gene transfer vectors, recombinant cells, and transgenic animals containing nucleic acid encoding invention modified ecdysone receptor.